Killer wasps drive flies to medicate their kids with booze

Kills some offspring, but kills even more parasites.

Our interaction with medication involves many conscious decisions. We recognize a threat (either existing or pending), diagnose the nature of the threat, and then choose a remedy to match it. But does medicating really require that level of decision-making? A study in today's issue of Science suggests fruit flies are capable of medicating not only themselves but their offspring as well. And their medication of choice? Alcohol.

The threat for these flies is any of a number of small, parasitic wasps. These wasps lay eggs on the larva or pupa of the flies, and their offspring feed on the animal internally, often killing them in the process. (Flies have larval stages, during which we call them maggots, and pupate just as butterflies do before emerging in their adult form.) Once infected, there isn't much one of the larva can do to get rid of the parasite.

Its one option: booze. Fruit flies, as their name implies, like to dine on fruit, especially during the larval stages. In many cases, that involves ingesting the alcohol that's produced by natural fermentation of rotting fruit (this can approach 20 percent alcohol content). Some species of flies have developed the ability to tolerate this alcohol as they chew through the fruit as maggots. But for most of the wasp species, even moderate levels of alcohol are toxic.

To demonstrate that this is an effective defense mechanism, the authors of the new paper set up flies to lay eggs either on a regular food source or one that was laced with six percent alcohol. With no wasps present, the eggs laid on alcohol-free food did better, producing adult flies at a higher rate. But when parasitic wasps were added the results were reversed—the booze-laced food start producing adult flies at a higher rate. Although the alcohol remains somewhat toxic, it still pays off when wasps are present.

Maggots eat pretty much whatever food their eggs were laid on, though, so they don't have much choice in ingesting alcohol. Their mothers, on the other hand, do have a degree of choice. The authors set up a situation where the females were given a choice of two food containers: one normal, one spiked with alcohol. When there were no wasps around, the flies preferentially laid eggs on the alcohol-free feed. When wasps were put in the cage, the preference shifted, with more eggs being laid on the booze-soaked meal. The flies would keep opting for the alcohol right up to a 16 percent concentration, about the limit found in naturally fermenting fruit.

How do the flies manage this trick? The authors of the paper showed they're remarkably specific. Placing male wasps (which are not a direct threat) in the cage didn't elicit any change in behavior. But the flies were able to recognize and respond to three different species of wasp. A functional visual system was required to respond to the wasps, and once the threat was recognized, a long-term memory was formed that kept the flies laying eggs on alcohol-laced food for days afterwards.

By checking a variety of Drosophila species, the authors showed the behavior correlated strongly with the evolution of alcohol tolerance—if the fly species couldn't tolerate booze itself, it wouldn't lay eggs on the alcohol-soaked food whether wasps were around or not. The authors note that a gene involved in forming the associated long-term memories is also a key regulator of a gene involved in alcohol tolerance, which suggests the evolution of the behavior and the tolerance might be coordinated.

Are the flies medicating their offspring? By the authors' definition—"the prophylactic (pre-infection) or therapeutic (post-infection) use of substances found in the environment to combat infection"—the answer appears to be yes.